1. Field of the Invention
The present invention pertains to the recovery of vapors of volatile substances. More specifically, the invention pertains to methods and apparatus for recovering the vapors of volatile substances from exhaust of storage tank vents, tank truck vents, industrial or chemical processes, and the like.
2. Brief Description of the Prior Art
In handling of volatile substances, such as fluid hydrocarbons, two phases are generally present: a condensed or liquid phase and a vapor phase which is generally in equilibrium therewith. The vapor phase creates a problem in handling and particularly in storage or transfer of the substance. For example, as a storage tank is filled with the volatile substance, vapors, which may also be mixed with air, are displaced by the substance being added to the tank and unless the tank is designed for high pressures, these vapors must be exhausted. For unpressurized tanks, there may also be daily exhaust, commonly called "breathing", due to temperature and atmospheric pressure changes.
The exhausting of vapors to the atmosphere results in a loss of valuable material. Furthermore, the exhaust vapors of most volatile substances contribute to air pollution and may be subject to state and Federal regulations, usually in the form of maximum allowable concentration of vapor emanating from the exhaust.
Several methods have been developed to reduce or eliminate vapor exhaust. Of course, one is the provision of tanks or containers of high pressure capabilities. Such methods are economically feasible only for small storage volumes. Probably one of the most common methods is to provide the storage tank with a floating roof which rises or falls with the increase or decrease in pressure. Floating roofs are relatively expensive and are only applicable to stationary tanks of cylindrical shape and with vertical axes.
In recent years, more sophisticated and economical methods have been developed. In one general group of methods, vapor is withdrawn from the storage vessel, compressed and cooled to produce condensate. The condensate is then returned to the storage vessel. An example of such a method is shown in U.S. Pat. No. 3,150,495. In another group of methods, liquid is withdrawn from the storage tank, cooled and sprayed into the vapor space of the tank primarily to cool the tank and reduce vapor pressures for filling or storage. Representative of these methods is the one described in U.S. Pat. No. 3,733,838.
There are, of course, several variations to these general methods. For example, U.S. Pat. No. 2,938,360 discloses a method in which tank vapors are fed into a condensor in which liquid, taken from the storage tank, is sprayed. However, there is no refrigeration of the liquid sprayed. A similar method is shown in U.S. Pat. No. 3,648,436 in which the entire liquid in the storage tank is refrigerated. Such a totally refrigerated system is relatively expensive to operate and to maintain. U.S. Pat. No. 3,714,790 discloses a relatively complex system involving refrigeration condensation and vapor compression.
As is evident from the many methods developed to date, no one method or system for recovering vapors has become universally accepted. Some are suitable for certain applications while undesirable for others. Many systems are relatively complex and expensive in construction and operation. With today's emphasis on conservation of our natural resources and abatement of pollution, more efficient and inexpensive methods are needed.